Aerated food products are widely known, for example food products like mousses, ice cream and whipped cream contain air bubbles which are stabilized in the food products. Gases commonly used for ‘aeration’ include air, nitrogen and carbon dioxide. Two factors are of importance in the development of aerated food products, and these are (i) the foamability of the product while introducing gas into the product during manufacture and (ii) the foam stability during storage, which is whether the gas bubbles tend to coalesce or collapse and whether the foam volume is retained during storage. Many additives are known to be included in the creation of stable foams, and these generally are compounds which are present on the gas bubble surface, which means on the gas-liquid interface during manufacturing of the foam. Known additives include proteins such as sodium caseinate and whey, which are highly foamable, and biopolymers, such as carrageenans, guar gum, locust bean gum, pectins, alginates, xanthan gum, gellan, gelatin and mixtures thereof, which are good stabilizers. However, although stabilizers used in the art can often maintain the total foam volume, they are poor at inhibiting the coarsening of the foam microstructure, i.e. increase in gas bubble size by processes such as disproportionation and coalescence.
Many of these food products additonally contain dairy protein as an ingredient. Dairy proteins like the caseins and whey proteins are among the most widely used food ingredients. Also other proteins like chicken egg protein or soy proteins are commonly used food ingredients. A property of many of these proteins is that they are surface active, which means that many of these proteins can act as emulsifiers or foam stabilizers in food products.
Due to the presence of the surface active proteins, these may interfere with the foam stabilizers present in the food product, effectively leading to unstable food product structure. Therefore in many cases however it is undesirable that the proteins are surface active. The presence of surfactants or proteins in the formulation may affect aeration due to competitive adsorption. Nevertheless due to its nutritional properties and for example texturising properties, food formulators still want to include the proteins in their food products. Hence there is a desire to use proteins which have strongly reduced surface activity.
It is also known that heat treatment reduces the ability of whey protein to act as foam stabilizer. Phillips et al. (Journal of Food Science, 1990, vol. 55, p. 1116-1119) disclose that the heat treatment of whey protein solutions reduces the foaming ability of these solutions. Dissanayake et al. (Journal of Dairy Science, 2009, vol. 92, p. 1387-1397) disclose a method to produce whey protein particles, which do not show foaming ability. The process involves a heating step at 90° C. for 20 minutes, subsequently a high pressure microfluidisation step at 140 MPa is performed, and this is followed by spray drying the obtained material.
Nicorescu et al. (Food Research International, 2008, vol. 41, p. 707-713) disclose that heating of solutions of whey protein influences the surface tension of these solutions: heating leads to a surface tension of about 40 to 50 mN·m−1, which means that the proteins are less surface active as compared to non-heated proteins. The thermodynamic affinity of protein aggregates towards air interfaces is weaker, however the proteins as disclosed are still surface active.
In a continuation of this work, Nicorescu et al. (Food Research International, 2008, vol. 41, p. 980-988) disclose that insoluble aggregates of whey protein apparently have the role of foam depressors.
EP 1 839 492 A1 discloses a method to prepare whey protein micelles by heating an aqueous whey protein solution at a pH between 3 and 8 to a temperature between 80 and 98° C., in the absence of shearing, followed by a concentration step. The last step is required to remove non-micellised material, or simply for concentration. The obtained micelles have a very small average particle size, and this material can be used as an emulsifier or as a foam stabilizer. This means that the micelles are still surface active.
WO 2008/046742 A1 and WO 2008/046732 A1 disclose aerated food products wherein the air bubbles have been stabilized by fibre material of which the surface has been modified by small particles to make the fibres surface active.
U.S. Pat. No. 4,855,156 discloses a frozen whipped dessert comprising non-aggregated particles of denatured protein. These particles are still surface active.
A common disadvantage of all protein particles and aggregates as cited, is that they are still surface active. Hence when preparing an aerated or foamed food product containing proteins, these protein particles will be present at the gas bubble interface. The surface activity of such particles may be relatively low, however as proteins are generally present at relatively high concentrations in compositions like food products, the number of particles at the gas bubble surface will be relatively high. Hence such particles may interfere with stabilizers present at the gas bubble interface, and compete for space with such stabilizers at that interface. As the number of protein particles is generally high, the protein particles will win such competition, and as they are not very strong gas bubble stabilizers, this will lead to instability of the gas bubbles and collapse of the aerated structure of the composition.